A. Field of the Invention
The present invention relates to communication systems, and in particular, to scrambled voice communications.
B. Problems in the Art
Radio frequency communication is proliferating. Examples include overland delivery systems, law enforcement and military networks, dispatch systems (e.g. taxis), inter-warehouse communication, and in-house security. The ability to wirelessly communicate over distances is advantageous and valuable for many applications.
Technology has produced, for example, hand-held battery powered transceivers which provide good audio quality and large area coverage. However, transmissions from these transceivers can be received by third parties.
A problem for a substantial number of radio frequency communication applications is, therefore, lack of privacy. This, in some instances, also involves security considerations.
This problem is widely acknowledged in the art. A variety of attempts have been made to address this problem. One principal method is to modify the radio frequency transmission so that it is unintelligible to eavesdroppers. The terms "encryption" and "scrambling" are used regarding these methods.
A discussion of these terms and some current systems and methods for "encryption" and "scrambling" can be found at, for example: Mailey, T. P. "How Scramblers Protect Mobile Communications", Cellular and Mobile International, Spring 1992 (pages 44-50); Kelley, S., and Wallace , H., "Split-Band Scrambling Furnishes Voice Security", Mobile Radio Technology, May 1988 (pages 18-28); McKernan, E. J. and Scott, B., "A Rolling Code Scrambler Gathers Diverse Functions", Mobile Radio Technology, Jul. 1989 (pages 42-58). The above listed articles are incorporated by reference concerning their background information.
These articles make it clear that extremely high security usually involves digital encryption techniques. The difficulties with these techniques include complexity and cost as well as the need for separate and costly equipment. Audio quality and limitations on distance for such communications may also come into play. The lowest cost and perhaps simplest techniques, such as voice scrambling using frequency inversion, can prevent casual eavesdropping, but those with sufficient skill and equipment can quite easily decode such scrambling methods.
One method which is generally seen as a reasonable compromise is voice scrambling of the analog signal containing the voice, using time-varying pseudo-random spectral modification of the voice signal. Its advantages include a better level of security than simpler frequency inversion techniques, as well as being less costly than digital encryption methods It can also be easily incorporated into many radio transceivers, including retrofitting existing units.
One requirement of such techniques is that there be synchronization between transmitter and receiver to allow scrambled communications to be de-scrambled. For example, before the receiver can decode it must know the scrambling sequence of the transmitter. It also must recheck synchronization or resynchronize from time to time.
This is not a trivial matter. Problems include loss of access to the transmitted signal due to vagaries in the radio frequency channel or interference, or even time delays in initially accessing the transmitted signal (for example, time delays involving cellular telephone voting systems and trunking grouping in multiple channel dispatch systems). The present state of the art therefore primarily uses an initialization packet and thereafter periodic (fixed or randomly spaced) bursts of information that contain synchronization code. These bursts are short broad band signals. Therefore, during the burst, the channel carrying the voice communication is interrupted to send data, including synchronization information. While this method does broadcast synchronization information periodically, the bursts occupy and therefore replace the voice for those periods. Other problems still exist.
First, if the receiver misses the initialization synchronization information, it must wait until the next burst. This time lag may result in loss of critical information. Secondly, if for whatever reason the receiver loses synchronization, a time lag will exist until the next synchronization information is received--again risking loss of information. Third, the use of periodic synchronization codes can affect audio quality. For example, sending entire initialization codes at certain intervals disrupts the audio portion of the signal, therefore substantially reducing audio quality. Also, there are practical limits on how frequently the bursts can be sent. There is therefore room for improvement in the art.
It is therefore a primary object of the present invention to improve over the problems in the art.
Another object of the present invention is to provide effective mid-level security scrambling without substantial negative effect on audio communication quality.
Another object of the present invention is to reduce or eliminate time lags relating to synchronization.
A still further object of the present invention is to provide continuous synchronization information as long as the transmitted signal is present.
Another object of the present invention is to provide immediate entry synchronization and to reduce or eliminate late entry synchronization problems.
Another object of the present invention is to eliminate loss of information.
A still further object of the present invention is to provide a cost effective, economical and efficient system and method of such radio communication.
Another object of the present invention is to provide a system and method which is relatively easily installed in transceivers, receivers, or transmitters, including retrofitting.
These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.